1. Field of the Invention
The present invention relates to a resource allocation and indexing method for orthogonal frequency division multiplexing (OFDM) symbol regions and frequency of a signal transmitted on downlink in a cellular OFDM wireless packet communication system.
2. Discussion of the Related Art
FIG. 1 illustrates an example of a conventional wireless (mobile) communication system. The wireless (mobile) communication system 100 includes a plurality of Base Stations (BSs) 110a, 110b and 110c and a plurality of User Equipments (UEs) 120a to 120i. Each BS 110a, 110b or 110c provides services to its specific geographical area 102a, 102b or 102c, which may further be divided into a plurality of smaller areas 104a, 104b and 104c. In a downlink, a transmitting end includes the BS and a receiving end includes the UE. In an uplink, the transmitting end includes the UE and the receiving end includes the BS.
When transmitting/receiving a packet in a mobile communication system, a receiver should inform a transmitter as to whether or not the packet has been successfully received. If the packet is successfully received, the receiver transmits an acknowledgement (ACK) signal to cause the transmitter to transmit a new packet. If the reception of the packet fails, the receiver transmits a negative acknowledgement (NACK) signal to cause the transmitter to re-transmit the packet. Such a process is called automatic repeat request (ARQ). Meanwhile, hybrid ARQ (HARQ), which is a combination of the ARQ operation and a channel coding scheme, has been proposed. HARQ lowers an error rate by combining a re-transmitted packet with a previously received packet and improves overall system efficiency.
In order to increase throughput of a system, HARQ demands a rapid ACK/NACK response from the receiver compared with a conventional ARQ operation. Therefore, the ACK/NACK response in HARQ is transmitted by a physical channel signaling method. The HARQ scheme may be broadly classified into chase combining (CC) and incremental redundancy (IR). The CC method serves to re-transmit a packet using the same modulation method and the same coding rate as those used when transmitting a previous packet. The IR method serves to re-transmit a packet using a different modulation method and a different coding rate from those used when transmitting a previous packet. In this case, the receiver can raise system performance through coding diversity.
In a multi-carrier cellular mobile communication system, mobile stations belonging to one or a plurality of cells transmit an uplink data packet to a base station. That is, since a plurality of mobile stations within one sub-frame can transmit an uplink data packet, the base station must be able to transmit ACK/NACK signals to a plurality of mobile stations within one sub-frame. If the base station multiplexes a plurality of ACK/NACK signals transmitted to the mobile stations within one sub-frame using code division multiple access (CDMA) within a partial time-frequency region of a downlink transmission band of the multi-carrier system, ACK/NACK signals with respect to other mobile stations are discriminated by an orthogonal code or a quasi-orthogonal code multiplied through a time-frequency region. If quadrature phase shift keying (QPSK) transmission is performed, the ACK/NACK signals may be discriminated by different orthogonal phase components.
When transmitting the ACK/NACK signals using CDMA in the multiplexed form in order to transmit a plurality of ACK/NACK signals within one sub-frame, a downlink wireless channel response characteristic should not be greatly varied in a time-frequency region in which the ACK/NACK signals are transmitted to maintain orthogonality between the different multiplexed ACK/NACK signals. Then, a receiver can obtain satisfactory reception performance without applying a special receiving algorithm such as channel equalization. Accordingly, the CDMA multiplexing of the ACK/NACK signals should be performed within the time-frequency region in which a wireless channel response is not significantly varied. However, if the wireless channel quality of a specific mobile station is poor in the time-frequency region in which the ACK/NACK signals are transmitted, the ACK/NACK reception performance of the mobile station may also be greatly lowered. Accordingly, the ACK/NACK signals transmitted to any mobile station within one sub-frame may be repeatedly transmitted over separate time-frequency regions in a plurality of time-frequency axes, and the ACK/NACK signals may be multiplexed with ACK/NACK signals transmitted to other mobile stations by CDMA in each time-frequency region. Therefore, a receiving side can obtain a time-frequency diversity gain when receiving the ACK/NACK signals.
In downlink of an OFDM wireless packet communication system, transmit antenna diversity may be obtained using four transmit antennas. That is, two modulation signals transmitted through two neighbor subcarriers are transmitted through two antennas by applying space frequency block coding (SFBC), and two subcarrier pairs coded by SFBC are transmitted through two different antenna pairs by applying frequency switching transmit diversity (FSTD), thereby obtaining a diversity order of 4.
FIG. 2A illustrates an example of operation of a diversity scheme.
In FIG. 2A, one block indicates one subcarrier transmitted through one antenna, and f1(x), f2(x), f3(x), and f4(x) denote any SFBC functions that are applied to simultaneously transmit two signals through two antennas and to maintain orthogonality between two signals at a receiving side. Examples of the SFBC functions are as follows.ƒ1(x)=x, ƒ2(x)=x, ƒ3(x)=−x*, ƒ4(x)=x*  [Equation 1]
In Equation 1, indicates a conjugate, namely, a conjugate complex number of a specific complex number.
In FIG. 2A, ‘a’, ‘b’, ‘c’, and ‘d’ indicate modulation symbols modulated to different signals. By repetition of a structure in which SFBC and FSTD are applied within an arbitrary OFDM symbol transmitted in downlink as illustrated in FIG. 2A, a receiving side can apply a simple reception algorithm repeating the same SFBC and FSTD demodulation. Pairs of the modulation symbols (a,b), (c,d), (e,f), and (g,h) are coded by SFBC. In actuality, subcarriers to which SFBC/FSTD is applied do not always need to be successive in the frequency domain. For example, a subcarrier in which a pilot signal is transmitted may exist between subcarriers to which SFBC/FSTD is applied. However, if two subcarriers constituting a pair, coded by SFBC, are adjacent to each other in the frequency domain, wireless channel environments of one antenna with respect to two subcarriers are similar. Accordingly, interference between the two signals when the receiving side performs SFBC demodulation can be minimized.
As described in the above example, when applying the SFBC/FSTD antenna diversity transmission scheme using four transmit antennas in units of four subcarriers, a system structure for obtaining a diversity order of 4 can be simply implemented.
Meanwhile, a plurality of signals can be transmitted by code division multiplexing (CDM) in a manner of spreading one signal in OFDM downlink to a plurality of subcarriers through a (quasi-) orthogonal code. For instance, when transmitting different signals ‘a’ and ‘b’, in order to spread the two signals at a spreading factor (SF) of 2 by CDM, the signals ‘a’ and ‘b’ are converted into signal sequences (a·c11, a·c21) and (b·c12, b·c22) using (quasi-) orthogonal codes (c11, c21) and (c12, c22) of two chip lengths, respectively. The spread signal sequences are added to two subcarriers and modulated as (a·c11+b·c12) and (a·c21+b·c22). For convenience of description, a signal sequence spread at an SF=N will be denoted by a1, a2, . . . , aN.
To allow a receiving side to demodulate a signal spread through a plurality of subcarriers by despreading the signal, each chip of a received signal sequence should experience a similar wireless channel response. If four different signals ‘a’, ‘b’, ‘c’, and ‘d’ that are spread at an SF of 4 are transmitted through four subcarriers by an SFBC/FSTD scheme as shown in FIG. 2B, received signals in the respective subcarriers are as follows.Subcarrier 1: h1(a1+b1+c1+d1)−h3(a2+b2+c2+d2)*Subcarrier 2: h1(a2+b2+c2+d2)+h3(a1+b1+c1+d1)*Subcarrier 3: h2(a3+b3+c3+d3)−h4(a4+b4+c4+d4)*Subcarrier 4: h2(a4+b4+c4+d4)+h4(a3+b3+c3+d3)*  [Equation 2]
In Equation 2, hi indicates fading of an i-th antenna. It is assumed that subcarriers of the same antenna experience the same fading and a noise component added at the receiving side is disregarded. It is also assumed that the number of receive antennas is one.
Spread sequences obtained at the receiving side after demodulation of SFBC and FSTD are as follows.(|h1|2+|h3|2)·(a1+b1+c1+d1),(|h1|2+|h3|2)·(a2+b2+c2+d2),(|h2|2+|h4|2)·(a3+b3+c3+d3),(|h2|2+|h4|2)·(a4+b4+c4+d4)  [Equation 3]
To separate the spread sequences obtained at the receiving side from signals ‘b’, ‘c’, and ‘d’ by despreading using a (quasi-) orthogonal code corresponding to a signal ‘a’, wireless channel responses to the four chips should be the same. However, as can be seen from the above example, signals transmitted by FSTD through different antenna pairs are (|h1|2+|h3|2) and (|h2|2+|h4|2) which are different wireless channel responses. Therefore, different signals multiplexed by CDM can not be removed completely during despreading.